The processes and fluxes that produce the distinct compositional structure of Earth’s continental crust by subduction remain controversial. The rates of oceanic crust production, in contrast, are well quantified and are generally believed to be faster than those responsible for building magmatic systems in subduction settings. Here we show that a recently recognized crustal section, the 30-km-thick Ordovician Sierra Valle Fértil–Sierra Famatina complex in Argentina, was built magmatically within only ~4 m.y. More than half of the crustal section represents additions from the mantle, and is preserved as mafic igneous rocks and maficultramafic
cumulates; the remainder is tonalite to granodiorite with evidence for widespread assimilation from highly melted metasedimentary units. U-Pb zircon geochronology reveals that the construction of the arc was not a simple bottom-up construction process. This continuous exposure of the arc crust allows the quantification of field constrained magmatic addition rates of 300–400 km3 km–1 m.y.–1. These rates are similar to those determined for modern slowspreading mid-ocean ridges and are of the same magnitude as magmatic addition rates required to build certain large segments of the continental masses such as the Arabian-Nubian shield,
among others. The implication is that significant convective removal of arc roots is required over time in order to build the modern continental crust via subduction-related magmatism.

Figure 1. A: Location map of the study area in central South America. The Sierra Valle Fértil and Sierra Famatina ranges (part of the larger Famatinian arc of the western Gondwanan margin) are shown. To the west, across a major tectonic boundary, is the Laurentian Cuyania terrane, which accreted to South America at the end of Famatinian magmatism. B: Simplified map of the central part of the Sierra Valle Fértil range, showing the main geologic units of the Famatinian arc exposed in the study area. The geologic map is a general view of the arc, with the western parts containing the deeper exposures (Pankhurst et al., 1998). Location of the new age-determined samples and the ages are shown in the figure.